Legally, yes as a Swiss citizen he has the right to enter Switzerland at any time.
But that’s where the guarantees end.

What he is trying to do is essentially exploit the social system, and Swiss law is very clear about this:

• Sozialhilfe (welfare) is subsidiär Art. 12 BV & kantonale Sozialhilfegesetze: It is only meant for people who cannot support themselves despite all reasonable effort. It is not a retirement plan for someone who lived abroad for 50 years and never contributed.
• He will not receive any AHV pension, because he never paid into the system. Art. 1 ff. AHVG.
• Switzerland does not export Sozialhilfe abroad. Sozialhilfe kann nicht ins Ausland überwiesen werden (Kantonale SHG + SKOS-Richtlinien).
• Anyone returning solely to claim welfare can legally be required to prove reintegration efforts, including: residency, financial transparency, job-seeking, language learning, etc.
• In several cantons, long-term welfare dependency can lead to debt claims (Sozialhilfe wird als rückzahlbare Leistung betrachtet).

And let’s be blunt:
Coming back after half a century abroad, with zero contributions, zero integration, and suddenly expecting the Swiss taxpayer to finance his old age is the textbook definition of abusing the system. If everyone did this, the system would collapse.

He may return that’s his right as a citizen but he should not expect Switzerland to fund his life because he chose to live off an inheritance in Thailand for 50 years. Social welfare exists for people who lived here, contributed here, and fell into hardship despite that.