Would Frank not be the worst friend in the world?

I agree with you, but I think it's also a matter of Frank being a younger guy. Roy has been married, divorced, might have had a child (which he doesn't find out till later was miscarried). During the movie he also shows a lot of pent-up regret and a growing disinterest in conning (especially when he speaks with Angela after revealing he's a con artist--he says he doesn't like his job and doesn't want her do get into it). Essentially, Roy has lived his life and has had his fair share of issues. Frank's a younger guy, single, doesn't seem to have any of those familial/marital problems, and he seems obviously unaffected by the questionable ethics of his job. Perhaps he's a reflection of a younger Roy? Which is why Roy ended up divorced in the first place. Remember in his second session with Dr. Klein, he reveals he hit his wife and was hardly sober. Seems like a completely different person from the Roy we see in the film.

I think like Roy, though, Frank is also a multi-layered character. He did show some signs of contrition in his farewell letter to Roy. He says (paraphrasing from memory): "You told me if I ever had a chance to take something big, I should take it." I think I do remember him apologizing in his own kind of way; sort of like an 'all is fair in love and war' sort of explanation. It kind of reminds me a lot of old Chinese/Japanese films where the master takes a new student under his wing, and eventually, the student kills the master; the master knows it's a part of the cycle of keeping his martial art going, so he accepts his fate. In a way, that happens on two levels here with both Angela and Frank 'killing' Roy.

I think the beautiful aspect of Matchstick Men is that, as heartbreaking as the end is for Roy (at least, in terms of not actually having a daughter), it's necessary for him to be swindled by his closest friend and 'daughter' in order to have an epiphany: what he wants in life isn't money, it's a family. This is something he didn't realize until after thinking he had a daughter.

Generally speaking, criminals are not good people to be friends with.

Well there is the whole Brutus and Caesar thing....Et tu, Brute?