Airtel wouldn't expect you to use someone else's number, so if your friend is also logged in on same app, maybe they do a check and prevent additional devices to login - ofcourse it may not be just one device as one user may have phone + tablet etc., but it may be doing some checks to prevent "misuse" (according to them)
Airtel might be putting additional controls in place to detect misuse of logins - this is obviously a loss of revenue for them, and so some changes might have been done in recent past. People familiar with such inside information may be able to confirm.
