She thinks her husband’s beard is untidy; is she sinning if she asks him to remove it?

It is haraam for the man to shave his beard or shorten it,
because of the evidence which enjoins letting it grow. 

See the answers to questions no.
110642 and
100909. 

What the believer is required to do is submit to the command
of Allah, may He be exalted, and the command of His Messenger (blessings and
peace of Allah be upon him), and not feel any reluctance in doing so,
because Allah says (interpretation of the meaning):

“But no, by your Lord,
they can have no Faith, until they
make you (O Muhammad صلى الله عليه وسلم)
judge in all disputes between them,
and find in themselves no resistance against your decisions, and accept
(them) with full submission”

[al-Nisa’ 4:65]

“It is not for a believer, man or woman, when Allaah and
His Messenger have decreed a matter that they should have any option in
their decision. And whoever disobeys Allaah and His Messenger, he has indeed
strayed into a plain error”

[al-Ahzaab 33:36]. 

The husband has done well to let his beard grow without
paying any attention as to whether it is attractive or not. 

It is not permissible for the wife to ask her husband to
shave it or shorten it, and she is sinning if she does that, because it is a
call to sin and going against sharee‘ah. 

Our advice to her is to accept this matter with an open
heart, and to understand that obeying the command of Islam is something in
which there is goodness, prosperity and success; the one who lets his beard
grow is emulating the leader of the sons of Adam [the Prophet – blessings
and peace of Allah be upon him] and the Prophets who came before him, and
the Sahaabah and Taabi‘een and those people of goodness and virtue who
followed them. Shaving the beard only became commonplace in recent times. 

Something that will help her to accept this is to understand
that Allah tests His slaves with whatever He wills of good times and bad, as
He says (interpretation of the meaning): “Everyone
is going to taste death, and We shall make a trial of you with evil and with
good. And to Us you will be returned” [al-Anbiya’ 21:35].
If she is put off by the beard, or she does not like how her husband looks
with it, let her be patient and put up with it, and she will be rewarded in
sha Allah; it is sufficient for her to know that her husband wants to obey
his Lord and follow His command. 

But we also advise the husband to pay attention to adorning
himself for his wife with whatever he can of permissible things that are
appropriate for men. This is one of the rights that she has over him, just
as it is one of his rights that she should adorn herself for him, because
Allah says (interpretation of the meaning):  “And they (women) have
rights (over their husbands) similar (to those of their husbands) over them
to what is reasonable” [al-Baqarah 2:228]. Ibn ‘Abbaas (may Allah be
pleased with him) said: “I like to adorn myself for my wife, as I like her
to adorn herself for me, because Allah, may He be exalted, says
(interpretation of the meaning): ‘And they (women) have rights (over
their husbands) similar (to those of their husbands) over them to what is
reasonable’ [al-Baqarah 2:228]. But I do not like to take all my rights
from her, because Allah, may He be exalted, says (interpretation of the
meaning): “but men have a degree (of responsibility) over them”
[al-Baqarah 2:228]. 

Narrated by Ibn Abi Shaybah (5/272) and al-Tabaraani in his
Tafseer, 4/532. 

Al-Qurtubi (may Allah have mercy on him) said: 

The scholars said: With regard to men’s adornment, that
varies according to their situation and they do that according to what is
appropriate and brings harmony. Some kind of adornment may be appropriate at
one time but not at others, or one kind of adornment may be appropriate for
young men and some other kind for old men but not for young men. Do you not
see that if the old man or the middle aged man trims his moustache, that
befits him and is an adornment for him, but if a young man does that, it
will look silly, because the beard has not grown yet and if he trims his
moustache when it first appears, his face will look silly, but when his
beard grows, if he trims his moustache, that is an adornment for him. It was
narrated from the Messenger of Allah (blessings and peace of Allah be upon
him) that he said: “My Lord commanded me to let my beard grow and to trim
the moustache.” Narrated by Ibn Jareer and others; classed as hasan by
al-Albaani. The same applies to clothing. All of that is part of fulfilling
his duties towards his wife, so a man should do that on the basis of what is
appropriate and brings harmony, and he should adorn himself for his wife in
a way that makes her happy and keeps her chaste so that she is not tempted
by other men. 

The same applies to kohl for men; there are some for whom it
is befitting and others for whom it is not befitting. As for perfume, using
the miswaak, cleaning the teeth, removing excess hair, circumcision and
clipping the nails, this is appropriate for all. End quote. Tafseer
al-Qurtubi, 3/124. 

To sum up: It is not permissible for the wife to ask her
husband to remove his beard, because that is telling him to disobey Allah.
But the husband has to adorn himself for his wife by combing his beard,
keeping it clean and perfuming it, and using whatever permissible means of
adornment are appropriate for him. 

And Allah knows best.