I've never thought of Mock Orange as a shrub that likes full shade. In fact, I have 2...one is in full sun and the other is in part shade. (I'm not sure what zone you are in but I'm in zone 5a) Depending on the cultivar of your Philadelphus, I wouldn't put it too close to anything...they can get BIG and will need constant trimming.
Just my 2 cents worth...