Teams Client, Guest User access, Separate Tenant

Check Entra ID, Users, search his email. Guest type, UPN ending #EXT#@tenant-business.onmicrosoft.com, means he's properly invited. Not there means he never was, and that's your answer right there.

Works on web, fails on desktop switcher, is almost always one of these:

Cross tenant access settings aren't allowing B2B inbound on your side, or outbound on his. Both need to agree.

His home tenant blocks external Teams orgs on desktop. Common default.

He's logging in with the #EXT# UPN and a password. Won't work, those accounts don't have one. He needs to log in through his home identity.

Verify the guest object exists. Check External Identities, Cross tenant access settings, set inbound to Allow for his domain. He needs to check outbound on his end.

His login flow: Teams desktop, profile picture, Add another account, sign in with his normal home credentials. No separate password. Your org should then appear in the switcher.

Two E3 tenants syncing is a good opening for a cross tenant access review and a B2B governance framework. Also worth raising guest lifecycle management via Entra ID Governance or PIM.

Don't let him try #EXT# credentials directly. Don't assume web access means B2B trust is configured. Don't skip his home tenant's outbound settings, that's the half you can't see.

What does his entry show in your guest list?