But probelm is that he has logged in to some other device as well and do not know which is that device, so the sms may never arrive and only the support can help to identify and logout all the unknown devices.
Also, I don't you how quick and efficent is the telegram support ? If thier response time is low, then it may take few days for them to see the issue and come to the conclusion. And yeah this is a frustration time when you are unable to login to telegram.
Was going to recommend him same, about using VPN, but it seems his other telegram ID working perfectly fine. So, it’s not VPN or connection issue with his account. It’s probably his mobile network issue, might have banned specific number where to receive messages. Telegram support sucks actually, they never reply you. But he can try connecting with them, hopefully they’ll help him.