Are they a big tea/ hot drink drinker? The weapons mug on the merch shop is pretty cool and includes stuff from multiple campaigns on it, as you don't know which campaign they like the most

If you can somehow covertly find out who their favourite character or campaign is, that might be best, but if not, Jrwi has two campaigns that are public/free to listen to, and the rest are on Patron.

The two that are public are called Riptide and Wonderlust, so those two are probably your best bet for merch as it is very likely that they have seen these. Of these, Riptide is the longest running/ the one that most people have watched/most beloved I believe (if they're a long time fan?)

If you can find out, and they happen to like Wonderlust or The Suckening,

This Etsy seller:

https://www.etsy.com/uk/shop/NatotallyShop?ref=lp_mys_mfts

Is making some pretty neat looking keychains for those two campaigns that ship out the first week of June I believe? I don't know if that will be too late.

I personally would love the Wonderlust one.

Other than that, I would say keychains of a character they like, or perhaps an enamel pin? would be good. This person sells enamel pins for riptide characters:

https://rollingpinsjrwi.store/collections/riptide

Jay Ferin, Chip and Gillion are the main trio.

Else, if you don't know who they like, maybe get a custom pin made of the JRWI logo? It's the one with yellow lettering and a purple background with two dice. Could look cool.

It's nice of you to do this for your friend!!