The Duchess' dad, Thomas Markle, was in Rosarito, Mexico Monday stopping off at a Starbucks drive-thru -- and seemingly walking away with a Frappuccino in hand, whipped cream and all. Mind you ... Thomas just underwent heart surgery last week.

Now, we're not doctors or anything ... but for whatever reason, it doesn't seem like a good idea to slurp up one of Bucks' sweetest, calorie-packed beverages on the heels of a heart procedure -- not to mention a very recent heart attack, too. That's just us, though.

Maybe he's just celebrating the fact that Meghan's wedding went off without a hitch, despite the fact he couldn't be there. Consolation caffeine hits the spot, apparently.