Munna are rather easygoing pokémon, and thus, they can likely get along with a laundry list of others. Still, I would be wary of pairing yours up with dark-types with more energetic or violent reputations (such as sneasel) or ghost-types known for being mischievous or prone to feeding off of dreams (such as misdreavus). The drowzee line also don’t quite get along with munna. It’s not that a munna wouldn’t be friendly with a drowzee, of course; it’s more that drowzee often respond coldly to munna. This is the only psychic-type that shouldn’t be put with munna, as the others respond rather well to them.
Meanwhile, houndour and houndoom, despite being dark-types, are actually often paired with munna, as they’re highly loyal and easy-to-train guard dogs who aren’t affected by munna’s dream mist. (In other words, they make excellent herding dogs.) Alternatively, slurpuff (not so much swirlix, as swirlix aren’t often strong enough) and lillipup both possess abilities that can resist munna’s sleep powers. So does noctowl, which some breeders also use, as noctowl can herd stray munna that happen to be levitating.
Conversely, many breeders like to pair munna off with pokémon that could benefit from munna’s dream smoke, such as psyduck or other pokémon that are often stressed. In fact, some munna breeders foster retired nursing chansey, wigglytuff, audino, and comfey before they’re sent off to permanent homes, just to help them readjust to a less stressful lifestyle than that of the pokémon center.
Best of luck to your friend!