Last 3 year in a row I've taken my cruiser to Puerto Vallarta
Last 3 years in a row I've taken my cruiser to Puerto Vallarta, Guadalajara, and Michoacan.
Never had any problem.. Even had a major mechanical issue along the way in the alleged worse part of Mexico "sinaloa"
https://forum.ih8mud.com/az-copper-state-cruisers/601702-replace-spindle.html
Have been down 3 times in the Cruiser!

And just went down for the Xmass break, on my motorcycle! BMW K12RS...

Everyone told me don't take a flashy bike like that: "you'll get raped, killed, run-over, kidnapped etc.. etc... and not necessarily in that order " someone even told me they'll steal my liver for black market organs!
Well besides having a weaker liver due to the excessive beach drinking.. I HAD A BLAST !!! 1,400miles in 3 weeks
Again no problem even on the bike!

Am I lucky??? Who knows.. but I never had even a hint of a problem I even got a flat on the bike.. and after being snubbed at the local Harley dealer.. the Yamaha dealer patched and balanced my tire for $5 dollars (tip for the mechanic) after the owner of the Yamaha dealer ooogled over my beemer and offered me $13,000 Pesos for it.. $10K U.S.

and refused to charge me for the repair saying.. It's free but you can tip my mechanic!

I have only meet helpful nice and very very cool people along the way..

(Except for those Harley Shop guys

I guess they are just trying to be like their American colleagues..

)
Best advice I can give you is talk to people .. Be nice.. and have a beer or margarita..
I personally prefer the tollways vs the free highways, just because they are generally shorter and faster, but I have nothing against the small roads especially if you are not in a hurry and want to see the countryside, stop in picturesque villages and towns and or sample local street food

BTW: your buddy with the GS will have an absolute blast.. the only BMW bikes I saw down there where all the 650GS's or 1200GS.. The GS is made for Mexico tell your buddy to watch out for TOPES!!! I went unsupported this last time on my bike, but with a support vehicle it should be easy!
Here's some pictures of my other addiction!