Credit Card Debt Question

Generally in credit card statements does it not show where the money came from?

No. Obviously, the credit card issuer will have records, but that information is not generally reflected on the statements. Of course, I'm not familiar with every credit card issuer's practices, so I can't categorically rule out the possibility that some of them include that information on statements, but it's certainly not the general practice in the industry.

Bottom line: She'll have to prove the obligation to the satisfaction of whomever administers the estate of the deceased ex-BF (if any administration actually occurs) or sue the estate, in which case she'll have to prove the obligation to the satisfaction of the small claims judge.
I'm going to preface this by saying that we're really just getting a vague second-hand retelling of a confused story.

True. So my next comment is just a guess.

The OP implies that the GF agreed to transfer the title to the ex. If the GF hasn't been paid in full, then why would she do that?

Possibly because she was never on the title. The BF bought the bike in his own name with a down payment from the credit card and financed the rest. The finance company either held the title or put a lien on the title and when the loan was paid, released the title or the lien to the buyer. The GF would have had no say in that and just got stuck with the balance on the credit card.

Is it something we can take small claims court about?

If you did, you would sue "The Estate of ________." The problem there is that the estate may have no money left after paying secured debts and priority debts. You might not find that out until after you win a judgment (if you win) and the judgment ends up being uncollectible.

She trusted him way to much for the short span they were together.

An important life lesson from the school of hard knocks: Never get financial involved with boyfriends/girlfriends.

You should follow that rule, too, lest something similar and costly happen in your future.