You could go with him to get the Gold card advantages, but that's as far as it goes.

For codeshares versus prime - as a general rule it's best to avoid codeshares unless there is a clear advantage from the codeshare, it's a complication on the whole, particularly if there are changes to schedule, voluntary or otherwise. But occasionally there are very specific advantages to being on a codeshare and you've picked up a few in the post above, such as using the e-Voucher. The ones that are perhaps less clear include that you have a slightly stronger EC261 protection on the DOH-JRO sector as a BA codeshare (though I wouldn't want to overstate that); and for all the moans on here are about BA's customer service and contact centres, some of the tales on the the QR forum can be pretty grim. A BA codeshare on Qatar means that BA holds the ticket as an agent.