My personal experience is mixed at LHR, and both occured holding the same status (Emerald, so presumably preferential over your friend's situation). A problem is that baggage handling at LHR is by BAA (or rules are set by BAA) and not the airline, this has resulted in a strict per piece weight limit for "health and safety" reasons, independent of the airline limit (if any). I think this is 32kg per bag, but have heard of some airlines enforcing a 23kg limit. The airline has no leeway around this, no matter what (the baggage handlers will refuse to load the overweight bag). Otherwise I have found CX to be much better than BA for weight limits out of LHR, so that is good news. But all airlines have cracked down on collecting excess baggage fees, making it much harder than a year or 2 ago. Be prepared to pay a few hundred dollars if there is an overweight fee.