Playing with a drummer, I would want at least 200 watts. But as others have said, tell him you need to try it to see if it is enough.
I think it's weird that people are like "if you're playing with a drummer, get at least 200W," though the Rumble 200 is only putting out about 140W through its stock speaker and doesn't get 200W unless hooked up to an extension cabinet, whereas Rumble 100 is putting out 100W through its internal speaker all the time. There is 40W difference in wattage in their combo format, it's not large. If you're using the amps as combo amps, the difference between Rumble 100 and Rumble 200 isn't wattage, it's mostly about the speaker, that you have a 15" instead of 12" driver.
And it's not as though you can't gig with a single 1x12" cabinet either. People swear by their 200W and 250W mini heads these days, I have played a gig (w/ distorted guitars and medium/hard hitting drums) with a BH250 and an 8 ohm cab, which is probably only putting out about 125-150 watts at 8 ohm. Was I high on the volume knob? Yes, but I got through the night with no sound reinforcement on the bass rig. I have rehearsed with a single Rumble 1x12" cab more times than I care to count, and it's absolutely fine, I rarely went that high on the volume knob either.
If plenty of bassists are gigging regularly with a single 1x12" 8 ohm cabinet and 200W mini heads that only put out like 125W when connected to a single cabinet, why wouldn't a 100W 1x12" combo work for a lot of gigs, also?
I'm not saying that the Rumble 100 will cut it for every gig, like it won't work if you're Sabbath loud and you have no PA support, but I would like to think that it works for a lot of people a lot of the time. It's definitely less volume than a Rumble 200, I'm not disputing that. But it's not like it's half as loud. In their combo format, the Rumble 200 is a bit louder, but the Rumble 100 will hold its own, and with the whole math about power, efficiency, and perceived volume, there's not as much of a legit volume difference as one might imagine. Plus, with the XLR output and the way most large venues are these days, you only need it to cover the stage after all. It'll absolutely cover a small bar or any rehearsal space if you have a band that plays reasonably loud.
I've seen so many bands with 100W combo bass amps, and most such amps are relics from a time when amps and speakers just weren't as good as the compact stuff being made today. Not necessarily a good comparison because the V2 150W is closer to a V3 200W, but I saw a band play on a Rumble V2 150W combo the other day, no PA support. Bass sound was absolutely huge, and the volume knob was quite low.
If you can swing a good deal on a Rumble 100, hell yeah it's worth it. It's absolutely tiny and just loud enough to start gigging with. When you have bigger amps, you might still keep that thing around.