Hi, does he want strong sativa or indica? Bud or vape, edibles? I can speak on bud only as I don't like vape or edibles.
Has he tried the strain Stardawg? This is the only strain so far that I have found to be consistently strong with my high tolerance. And it is the only strain so far that gives me strong effects. I have tried nearly every strain on here, used to only smoke sativa, but it can make you a little crazy, cause sativas are more of an energetic head buzz, so switching to Stardawg, was a game changer, it's slightly more indica dominant 60% indica 40% sativa, I think. And no paranoia, or crazy head buzzing. Stardawg also is the best for sleep, a few joints and you are out like a light, before this effect, you get a nice sativa buzz, which is not the uncomfortable dizzy buzz that sativa dominants give you.
Also it is a good idea to take tolerance breaks, I find even a one week break does the trick, my last break was two years, but now I find that one or two weeks without smoking does the trick. I buy from RadarBreeder, he has Stardawg and Purple Stardawg, with the best prices and choice of next day tracked Royal mail, or regular 1st Class. Very good packaging in normal looking jiffy bag, no smell leakage. I have had many deliveries with zero problems. There are a couple of other vendors who have this strain, however, I found RadarBreeder to be the best for prices, packaging, and communication, they are very helpful and honest if you ask them about their strains.
But ofcourse every person reacts differently to strains, so in my personal experience, Stardawg is the ONLY one that I can get a decent effect from. I still waste alot of money trying new strains but as yet nothing comes close to the stardawg. Hope this helps a little :) Let me know if he finds any other strain potent, cause I am always on the look out for stronger, even tho I am conent at the moment with stardawg.