<< You could get the Menzies red/white... >>

Yes, that as well as several other Menzies are fine-looking tartans.

But the MOB wanted that specific mauve tartan.

Probably because it'd compliment whatever unsightly dresses she & bride selected for the bridesmaids.

For that matter, perhaps she plans for the bridegroom & groomsmen to be in same tartan. Geez - that would be one visual train wreck.