
Are they both entire and have either or both had seasons?
Even hormones aside, both being bitches of exactly the same age and a similar breed mix on the BC side, this is not entirely unexpected behaviour although naturally unwanted. If the other bitch has something like Akita in her cross then again, there is an additional issue with character because the breed is not known for it's tolerance of others - certainly of the same gender.
I'd continue to meet up for now, keep both girls on their leads, and have a good walk together in controlled surroundings - i.e. no other loose dogs darting about to over stimmulate either of yours. When they've had the initial excitement of being out and about dampened a little, then I'd say give them separate opportunities to have some off lead running. The dog on lead should ideally be distracted by maybe training with treats and the off lead dog interacting with it's owner with free training and treats or a toy.
I'd try for a while on a regime where they are together but not physically able to interact with one another and hopefully as hormones and this juvenile stage start to taper off they will be relaxed in one another's company and be able to resume off lead play but go slowly, avoid mishaps or threatening behaviour. Set up for success and it should hopefully work out for everyone.
Talk this over with your friend in depth so that she doesn't feel her dog is entirely being blamed (even if it is 100% it's fault it's her darling ;) ) and suggest that you work together on some of the ideas so that you can enjoy these meetings for years to come :) With the right approach by all parties it can work - one of mine was a little brat with similarly aged or younger ones and eventually accepted that there was no need to show off / posture at every intro and thereafter completely settled.
HTH, Teri