Can he afford an SLS? Or at least the AMG variety? Why merc though, tell him to the get the M3 coupe.
Before the Alpina, he was looking at an S or SL 65. Which I believe both cost more than the SLS. The SLS coupe, is a tad more, obviously.
But now, he just wants a simple cruiser. I don't think he's looking to break his neck with a 0-60 closer to 2 seconds. Like I said, he's had a thing for Mercs.
Another BMW is kind of out of the question. It'd make three and, like I said, my dad has had his eyes on an SL since I was a little kid. Same with a Rolls, believe it or not. He said he may not really drive it, but it would be his gift to himself when he reaches a certain milestone in his career.
If I could convince him (which I can't) to look at other things, I'd tell him to look at the Jaguar's XK or F-Type. I think both of them are beautiful cars and they sound amazing. It's hard to believe but there are not many nice convertibles in such a strict price range of the SL. Some are not quite up to the mark of the SL and then there are others that are in a league of their own. $100K is kind of no-man's land.