Posted: 29/12/2014 at 15:20
It's a big shame - but there may be a silver lining. Philadelphus sometimes throws up one long branch and the others are quite spindly in comparison. If you had asked me I would have advised you to cut that main branch hard back and then it would have thrown up several new branches and you'd have a well balanced shrub.
Now, without even having gone to the bother of asking, you're going to get that well-balanced shrub
You'll have to do without flowers this year, but maybe you can plant something nearby that will focus attention for this year - just think of it as an excuse to go to the garden centre