As long as he isn't hurting anyone or getting hurt himself, that's fine. I just like to see pictures of him happy. ...... I'd just melt if I met him..... never mind anything else.
But for someone (me) who was in a secret relationship (just after university) they usually end badly. Not saying he is or that MM is telling the truth or fibbing (not quite sure what to think of what she said before), just saying in general secret relationships don't end well. Mine the guy didn't want to say to anyone as him, me and his ex had shared friends and he didn't want to rub her nose in it. I thought everything was hunky dory as his family, sibling and partner knew we were a couple and we'd all been out together etc. World came crashing in when I found out not only was he still seeing his ex, she thought they were going to get married etc. I dumped him but he took it very badly turned up at my door etc a few times causing a scene then threatening me if I didn't take him back saying his ex was delusional (she was telling the truth that she'd been seeing him). So I can see what impact a relationship where you're not acknowledged might take, but guessing if you're involved with someone famous that's a risk you'd have to take.
I can't see Tom being a cheat though, not at all. But I do think if he's single he's entitled to some fun.