I have a question that if A (boy) wants get marry with b (girl) and B has no Muslim from her family she is alone Muslim from her all family. who can be wali for her ? If A's dad become wali for his son's bride (boy dad become wali). Is nikah valid??
The waly of a former non-Muslim lady who converted must be a Muslim. She should chose him and he can't be from among the two necessary witnesses for the marriage. Usually people in such cases chose the imam of a mosque as the hadith says:
Aisha and Ibn Abbas said: “The Messenger of Allah said: 'There is no marriage except with a guardian.' ”According to the Hadith of Aishah: “And the ruler is the guardian of the one who does not have a guardian. ” (Sunan ibn Majah)
And as the imam of the mosque is the highest authority of the local Muslim community (at least in non-Muslim countries) he should be the first choice of waly else one should chose in first place a higher rank person like a qadi who etc.
See also in islamqa with more details:
If the woman does not have any wali who can give her in marriage, then that role may be filled by the Muslim qaadi (judge), if there is one. If there is no qaadi, as is the case for Muslim communities living in the West, then the director of the Islamic centre may give her in marriage.
Note that in the hanafi madhhab (which you have tagged) an experienced lady doesn't need a waly and can marry herself.