No, I don't think he should be allowed to check in with you – on the same booking it’s not unreasonable…but otherwise you could just bring any number of random people with you. Last time this argument came up it got incredibly tedious incredibly quickly as usual so I’ll say no more than that
Lounge access should be OK because ba.com (and the BA rulebook) says same CARRIER not same flight. However, you may have to stand firm with this and politely make them check and/or show them the page on ba.com where it says this. Give me a minute, I’ll find the link. Edit:
https://www.britishairways.com/trave...e/public/en_gb
Originally Posted by BA
Please consult the table below to check your eligibility for lounge access. Lounge access is subject to capacity restrictions and guests must be travelling on the same carrier as the Executive Club member.
(I was at T4 F the other day getting an e-ticket receipt and the lady denied a Gold a guest since the guest was on a different BA flight. I wasn't going to butt in and interfere so just had a discreet, polite word afterwards and she was very friendly about it but also very surprised when I showed her and said she'd check with management and make sure her colleagues knew ^).