I really figured I would hear that a 45 is large to conceal.
No, It's not the cartridge but the firearm it's chambered in. Most 45's are larger framed than their smaller 9mm's and 40's.
There are slim, very concealable 45's on the market. Like the Springfield XDS and Kahr PM45. Both are high quality firearms and will serve him well. But if he were open to 9mm or 40s&w his options open up tremendously.
And don't let any one tell you that the 9mm is a horrible round for self defense. Ask them if they've ever been shot by one or shot any one with one using proper defense ammunition. It falls right between the 38spl and 357 magnum in terms of "power". As a military NATO round it sucks due to the use of FMJ or ball ammunition. But as a defensive round using modern hollow point ammunition it is highly capable.