Unfortunately there is no easy way to downgrade the app to a lower version. Best you can do is ask if he can download the modeler for 8.7 as well..
A more risky and technical alternative: You can try to open de mpr with SQLite and try to change the version to 8.5.1 but this might brick you application. (If you search for this on the forums you will find more questions regarding this issue).
Edit: I forgot, the newer mendix versions now come with a mprTool, this might help you. You can find it in your Mendix folder, e.g. Mendix\220.127.116.115\modeler\MprTool.exe (open the mpr in the tool go to edit metadata)
With kind regards,