I think a lot goes into this. how old was he w/ the first marriage (sorry, but a lot of first marriages are "starter" marriages. Being too young and unprepared for being an "adult".)? And so you know cheating was involved w/ the 2nd, but as both people cheated.... kind of makes me think something was inherently wrong w/ that relationship to start with.
But so much more is needed to be known about both of these marriages. Age, how long, state of mind, how it fell apart, how long ago, etc.
Also - how long has this couple been dating? How soon into it did he start talking marriage? If it's still a relatively new relationship, I think it's a HUGE red flag that he's talking marriage. However, if it's been awhile, then maybe not a red flag.