How did Tony recover the $50,000 that he lent to Artie Bucco?

You're mistaken about the deal. From the Sopranos wikia:

In Season Four, Artie approaches Ralph Cifaretto for a $50,000 loan. This money would let him act as shylock to Jean-Philippe, the brother of the new French hostess at Vesuvio, who needed $50,000 short-term to fund a business venture. Ralph denies Artie's request on the grounds that if Artie couldn't pay him back, he wouldn't be able to hurt Artie in revenge because of his close relationship with Tony.

Tony finds out, and is hurt that Artie didn't come to him first for the loan. Tony agrees to lend Artie the money on relatively generous terms (1.5% interest) and Artie, in turn, lends the money to the Frenchman on more strict terms (12.5% interest). Jean-Philippe defaults (as he was scamming Artie the whole time), and Artie, seeking to force payment, visits him to rough him up—only to get beaten up himself.

Despondent and unable to repay Tony's loan, Artie attempts suicide by overdosing on pills and alcohol, calling Tony before he loses consciousness, sobbing, "I love you and I'm sorry I let you down." Tony had been in a fragile emotional state to begin with because he had just learned that his ex-comáre, Gloria Trillo, had committed suicide—which he blamed himself for. Tony calls 911 but is angry when he shows up at the hospital because of Artie's lack of consideration for those around him in attempting suicide, asking him, "Suppose I come over to your house and find you dead? How am I supposed to feel?"

Tony tells Artie that he'll assume the Frenchman's debt (including the interest), and collect it himself, if Artie agrees to cancel Tony's $6,000 tab at the restaurant. Artie agrees, but suggests that Tony knew how the whole thing would play out from the beginning (Artie would get suckered and fail; Tony would benefit twice by taking collecting on Jean-Philippe's debt, and get his restaurant tab erased). Tony becomes irately indignant over this suggestion and storms out, telling Artie not to tell anyone about either their arrangement or the suicide attempt.

The two don't speak to each other for the rest of Season Four, and into Season Five; although, Tony still frequents Vesuvio during this timeframe.

Tony assumed the responsibility of collecting the debt. He taxed Artie 6 grand for that privilege. Ruthless yes but considering how Tony handled a comparable situation with his buddy who owned the sporting goods store in the past, Artie got off pretty light. As for the French guy who scammed Artie he probably ended up paying way more than the 50 grand as members of the Dimeo family (starting with Furio) showed up and bled him dry.

Tony himself states multiple times throughout the series that getting people in over their head in debt to him and then bleeding them dry is his bread and butter. It's possible they killed the French guy but more likely they beat him and collected payment under threat of death, harassing and extorting him for years if necessary. The show never states so definitively but I assume Tony ended up getting all of that money back plus vig ultimately making a healthy profit.