My sister recieved a marriage prospect which was supposedly good from all aspects and our family was looking forward for the wedding. But after a few days we recieved some pictures where the man was with his friends in several bars. His friends did drink alcohol but he didnt and the alcohol was obviously served on the table. What are we supposed to do in this situation? Is an istikhara valid here?

If there are strong indications that he didn’t drink, and he promises not to go there anymore, then she can proceed with the marriage proposal. But your family must make it clear that they will only accept if he agrees to stop hanging out with such friends at a bar.