what will you do if you saw your best friend's husband dating another girl?

It depends, it can go both ways. You can tell your friend about this or you can decide that it's not really your place to say anything and keep quiet. Either way, you are stuck between a rock and a hard place. Personally, I wouldn't say it directly because I don't know all the facts and it could lead to a misunderstanding if my facts are inaccurate. I'd probably start off my asking my friend about her husband, their relationship like I usually do and find a right time (if there is any) to bring it up.