All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The husband is obliged to spend on his wife according to his ability. Allah Says (what means): {Let a man of wealth spend from his wealth, and he whose provision is restricted - let him spend from what Allah has given him. Allah does not charge a soul except [according to] what He has given it. Allah will bring about, after hardship, ease.} [Quran 65:7]
The wife is not obliged to spend on herself even if she is rich. Rather, if she spends on herself while her husband is insolvent, she is entitled to ask him to reimburse her for whatever she had spent on herself (later), according to the most preponderant view of the jurists.
Therefore, if she spends money on her husband, she has the right to ask him to reimburse her for it when he becomes solvent if she did not give him the money as a donation.
A man marrying a second wife is permitted provided one fulfils the condition that Allah clarified in His Book; which is to be fair between his two wives. Allah Says (what means): {…then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one.} [Quran 34:3]
There is a difference of opinion among the jurists when the husband is poor and is unable to provide his family with the obligatory spending, whether or not it is permissible for him to get married: some jurists permitted this for him while some others forbade him from it. In general, some jurists are of the view that it is recommended for the husband not to exceed one wife if chastity is achieved by her only.
In conclusion, we advise the questioner –in case her husband marries a second wife –to fear Allah and be patient, and to fulfil her obligation towards her husband (his rights), and to ask him to fulfil his obligation towards her (her rights upon him). In case she is harmed by staying with him, or hates to stay with him, then she may ask him for divorce or Khul’.
We would like to draw your attention to two matters:
1- If the husband does not have money to spend on the people whom he is obliged to provide for, then he is obliged to seek to earn a living and fulfil this obligation.
2- Having children is a mutual right between the spouses; neither of them has the right to refrain from it without an Islamic legitimate reason.
Allah knows best.