One would hope that the
One would hope that the service advisor was trying to save you the cost of a tow or pick up service being aware you were not hundreds of kilometres away. Also one would hope and believe that in investigating the problem, even if they already had a fair idea because of previous similar experiences with this model,
that the remedial process would involve a system flush, and refilling with the correct coolant mix etc.
In this instance topping up with water to get to a close point of repair is extremely unlikely to cause any problem. However it should not be done as a routine process because it dilutes the correct mix ratios of whatever is used for coolant. Personally I usually carry a small reserve of the correct mix ( 2l) either on a long trip or have it handy in the shed at home when only travelling locally.
I hope BMW gave you a list of generally available additives you could use. Without that their advice is almost worse than useless. BTW I have had BMW bikes for years but would be unlikely ever to have a car. Actually, in many ways I think they have lost the plot in both directions, in many ways I would sooner have my older 50/60's bikes back.....