I have a Thai friend who is 22. He is positive for Covid. He is at home. There are no beds available in Bangkok. He is alone and being safe. I sent him money for food and drinks. He does not have a fever or cough. He does have no smell. Recommendations for giving him advice is greatly appreciated.

Treat whatever symptoms he has, take acetaminophen (I was surprised by this, I avoid Tylenol, my liver works hard enough as it is)
 

Stay hydrated, get lots of rest...Headaches was the worst part and being more sleepy than normal.

From the internet:

If someone is showing emergency warning signs, get medical care immediately. Emergency warning signs include:

Trouble breathing

Persistent pain or pressure in the chest

New confusion

Inability to wake or stay awake

Bluish lips or face

There is no doubt that the Delta variant is affecting much younger people than before. My brother's 11-year old grand daughter became infected and had to spend 4 days in hospital.

One thing is to get him oxymeter, so he can quickly monitor his oxygen level. Happy hypoxia is dangerous so early detection is crucial. Other than that, his young age will protect him, whats more important is for him to ensure he is not infecting others. 

Knowing when to seek help is also crucial. Im not sure how thailand is managing this but over here, those who are quarantine at home will have to update their symptoms daily on the app, and if experiencing things like difficulty breathing, and signs of blood clot (pain in legs, stomach, head etc), they need to go to emergency asap. Having oxymeter to know oxygen level is also helpful as anything below 95% require medical attention. But since the boy is young, i wont worry much on this. 

Also to be aware of is the long covid effect, which require doctor's check up from time to time after he is cured. Mental health is also crucial during this period, due to many reasons.

 

Vitamin D is being further studied for its effects on preventing and treating Covid. The following rationale is an excerpt from the US National Institutes for Health:

"Because the vitamin D receptor is expressed on immune cells such as B cells, T cells, and antigen-presenting cells, and because these cells can synthesize the active vitamin D metabolite, vitamin D also has the potential to modulate innate and adaptive immune responses."

Although the NIH says the evidence of its effectiveness is inconclusive at this time, further studies are being conducted.

Vitamin D deficiency is more probable when a person is not exposed to sufficient sunlight, which would be the case if they are quarantining at home. My primary care physician put me on 2,000 units a day to help maintain bone density.

I believe it's readily available in Thai pharmacies and stores like Boots and Watson's without prescription.  The cost should be quite reasonable.

I'd certainly second Spoon's recommendation of providing an oximeter.