What is the best visa option for a Swedish national married to a Thai citizen who wants to split time between Sweden and Thailand?

If you get the 1 year Multiple Entry, come to Thailand get 90 days of stay, make a border run and get a new 90 day stay period, can do another border run or Extension of Stay for 60 days.

he is married, get the Non Immigrant O visa based on marriage, if they only give the Single entry, come to Thailand get 90 days of Stay, apply for 60 days Extension of Stay at local immigration on basis visit family, then make 1 border run for a 30 day Visa Exempt Entry and if you like to stay longer apply for a new 60 day Extension of Stay.

Other (incorrect, but potentially viable) options:

Multiple Entry Tourist visa: Valid for 6 months, but requires a border bounce every 60 days. Same cost as a Non-O Multiple Entry tourist visa (I believe), and can only be acquired in his home country OR Perth, Aus.

Visa Exempt. Unlimited by air, twice by land. The cost is free, however there's no promise immigration will let him in. Each entry is good for 30 days, extendable by another 30 days for 1,900 THB. If he's rejected at the border, he'll have to wait at the immigration detention center until he can pay for his flight back to Sweden.

Single Entry Tourist Visa: Valid for 60 days, and can only be used once. A person can get "unlimited" visas, with unlimited defined by the consulate or immigration officer.

If your friend is willing to bounce border ever 90 days (or less), then a one year Non-O Multiple Entry visa based on marriage would be best. I believe that's the cheapest, and the one that allows your friend to enter and leave as needed.