They 200% have sleep apnea and a sleep study would allow them to get additional treatment, like CPAP/APAP/BiPAP. I'd recommend that they immediately consider some lifestyle changes as well as getting a cheap Fitbit to track their sleep quality (particularly REM sleep). The problem is that mouthguard is palliative. It doesn't address the root condition, which is inflammation. They'll get the mouthguard and continue to have sleep apnea. The mouthguard only masks the much greater problem.
There are about a thousand things that can cause apnea, but drinking, drug use, and late night snacking all contribute to it. I'd strongly recommend intermittent fasting, abstaining from drug/alcohol/tobacco use at night, and adding cardiovascular exercise into their daily routine.
In the short term, until the mouthguard is ready, I'd want to use one of these:
https://www.amazon.com/Sleeping-Stopper-Silicone-Breathing-Snoring/dp/B09KZT8PXY/
It's super simple, cheap, and it will prevent grinding and clenching, unless you take it out of your mouth during sleep (which some people do).