I can't find anything on the validation of this tool. It measures stress/strain/coping/support for family members of substance users (or those addicted to gambling).
As far as I can tell there is just one relevant publication: Orford, J. (2017). in Drugs: Education, Prevention and Policy, 24(1): 9-16.
It doesn't seem to have a lot about scale development, reliability, validity etc. though. My sense (and this is after a very quick search) is that the folks involved aren't really well versed in test development. I reckon they'd like your help!
The basis for the FMQ questionnaire is clear in the three questionnaires discussed in the paper you mention. The paper ends however,
"The proposed methods will require thorough testing in service settings; we are currently seeking service collaborators in order to carry out such tests."