My brother and sister are arriving at LHR on different flights and meeting there to travel onward together. She's arriving at T4 on MH and connecting to BA at T5.

What's a reasonable estimate of the time it would take her to get from T4 to T5 airside ready to go to the gate?

She'll be in Y, no status. We're trying to work out an agreed time and place to meet since neither will have mobiles that work in the UK.

Thanks!