He borrowed from his wife and wants to take a second wife

There is nothing to stop a wife asking for
her money back from her husband. This is a right that belongs to her. But we would like to point out a few things. 

1 – A wife should not make things hard for
her husband just because he wants to take a second wife. The basic principle is that people – above all husbands and wives – should cooperate in
righteousness and piety, and should not make things difficult for a person who seeks something that is permissible. 

2 – The owner of the money should not ask for
it back from a debtor who is in difficulty. Allaah says (interpretation of the meaning): 

“And if the debtor is in a hard time (has
no money), then grant him time till it is easy for him to repay”

[al-Baqarah 2:280]

3 – It is not permissible for a debtor who has the money to pay off his debt to delay paying it
off. The Prophet (peace and blessings of Allaah be upon him) said: “The rich person who delays paying back a loan is a wrongdoer.” 

Narrated by al-Bukhaari, 2166; Muslim, 1564. 

4 – We do not think that the husband should
use his wife’s money to take a second wife, because this is upsetting to her. He has to pay back the money to his wife, then if he has enough left
to get married, he may do so, otherwise he should not. 

And Allaah knows best.