He committed zina with a woman then incited her to get divorced from her husband, then he married her after they had both repented from this sin of zina, then they regretted what they had done

Firstly: 

What this man did of committing
zina with a married woman, then inciting her to leave her husband and ask
him for a divorce is a great crime and a major sin, and it is a blatant
immoral act which is indicative of the doer’s lack of religious commitment
and lack of respect for the limits set by Allah, may He be glorified and
exalted. That is because zina (adultery) is an abhorrent crime according to
all divinely-revealed laws, as is well known, and turning a woman against
her husband is a grave sin, in addition to the sin of zina. A stern warning
has been issued concerning it, as in the hadeeth of the Prophet (blessings
and peace of Allah be upon him): “He is not one of us who turns a woman
against her husband.” Narrated by Abu Dawood (2175); classed as saheeh by
al-Albaani in Saheeh Abi Dawood. Abu Dawood (5170) also narrated that
Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be
upon him) said: “Whoever turns a man’s wife or slave against him is not one
of us.” Classed as saheeh by al-Albaani in Saheeh Abi Dawood.

Shaykh ‘Abd al-‘Azeem Abaadi
(may Allah have mercy on him) said: “Whoever turns a woman against her
husband” by mentioning the husband’s bad qualities in the presence of his
wife, or by mentioning the good qualities of a “stranger” (one who is a non-mahram
to her) in her presence.

‘Awn al-Ma‘bood,
6/159 

He also said: “Whoever turns a
man’s wife against him” means by deceiving her or turning her against her
husband, and encouraging her to get divorced so that he can marry her, or so
that he can arrange her marriage to someone else, and so on.

‘Awn al-Ma‘bood,
14/52 

Secondly: 

What this woman did of
committing zina with that man is a grave sin and a betrayal of her husband
and a denial of his marital rights. The punishment of this crime for one who
is married – whether a man or a woman – is stoning to death, as is well
known from the mutawaatir Sunnah. 

Moreover, her asking her husband
for divorce when there is no good reason for that is haraam, and a stern
warning has been issued concerning that in the words of the Prophet
(blessings and peace of Allah be upon him): “Any woman who asks her husband
for a divorce when there is no good reason, the fragrance of Paradise will
be denied to her.”

Narrated by at-Tirmidhi, 1187;
classed as saheeh by al-Albaani in Saheeh Abi Dawood 

Thirdly: 

Some of the scholars are of the
view that if a man turns a woman against her husband, it is not permissible
for him to marry her; rather she becomes forbidden to him for eternity. This
is the view of the Maalikis. 

However the majority of scholars
are of the view that such a marriage is valid, even though they have sinned
by doing what they did. This has been discussed previously in fatwa no.
84849 

What appears to be the case in
this situation is that so long as this man regrets what he did, and the
woman regrets it likewise, and they have repented to Allah, may He be
glorified, from the sin of zina before marriage, then they both have to
repent again, sincerely and truly, from what they did of conspiring against
the first husband and breaking up his family. They should do a lot of
expiatory righteous deeds, as much as they can. Allah, may He be exalted,
says (interpretation of the meaning): “And perform As-Salat (Iqamat-as-Salat),
at the two ends of the day and in some hours of the night (i.e. the five
compulsory Salat (prayers)). Verily, the good deeds remove the evil deeds
(i.e. small sins). That is a reminder (an advice) for the mindful (those who
accept advice” [Hood 11:114]. And it was narrated from Abu Dharr (may
Allah be pleased with him): The Messenger of Allah (blessings and peace of
Allah be upon him) said to me: “Fear Allah wherever you are, and follow up a
bad deed with a good deed which will erase it…” Narrated by at-Tirmidhi,
1987; classed as hasan by al-Albaani in Sunan at-Tirmidhi. It was
narrated that Ka‘b ibn ‘Ujrah (may Allah be pleased with him) said: The
Messenger of Allah (blessings and peace of Allah be upon him) said to me:
“Charity extinguishes bad deeds as water extinguishes fire.” At-Tirmidhi
(614); classed as saheeh by al-Albaani in Sunan at-Tirmidhi. 

After having repented, they do
not have to separate from one another, as is the view of the majority of
scholars, including the Hanafis, especially as they have children who would
be adversely affected by such a separation. 

For more information, please see
the answer to question no. 216816

And Allah knows best.