Martian

You are correct, sir. Via translates to way, but is the sense of a road or path taken. This survives in our use of the word today (consider 'I travelled to Illinois via I-96). Furthermore, I'm pretty sure via when used as a noun qualifies as a place noun and therefore via sola could actually be translated as 'the lonely path' or 'the uninhabited path' which I don't think is the meaning you're looking for.

Note that it's sola here and not solus; remind your friend to conjugate.

If we wish to translate it as 'the only method' or 'the only technique,' we're still going to want the nouns modus or ars, respectively. Thus our options become:

modus solum ('the sole method' or 'the only method') or
ars sola ('the only technique')

Note as well that Latin used a Subject-Object-Verb word order. That means that properly our adjective should come after our subject, not before like in English. However, Latin's been dead for a couple thousand years now and the Romans weren't terribly picky about following their own rules anyway, so if you think it sounds better you can use solum modus, sola ars or sola via (if anyone asks, tell them it's poetic).

Because Latin is a dead language there is no real correct way to pronounce it. However, given your aversion to butts I will point out that the generally used pronunciation on ars leans more towards 'arrs' (as in multiple pirate exclaimations) rather than 'arse' (as in the thing you sit on).

And again, same warning. My Latin is terrible. If you want something that is definitively correct in both meaning and grammar you'll need to find someone more knowledgeable on the subject to help you.

__________________
I wake up in the morning more tired than before I slept
I get through cryin' and I'm sadder than before I wept
I get through thinkin' now, and the thoughts have left my head
I get through speakin' and I can't remember, not a word that I said

- Ben Harper, Show Me A Little Shame