Grass growing through rose bushes

Hi this may seem a simple problem, but I'm not sure of the best way to deal with it. My dad's garden has got a little unkempt and he now long weedy grass growing in the middle of his rose bushes. He now wants to dig the beds out properly and get rid of this. What is the most sensible way to do this. Just pulling at it won't sort it out permanently will it?

 

Welcome to Gardeners Corner

Pulling it will weaken it, but won't get rid of it, best wait till the spring growth and put some glyphosate gell on it, that'll take the roots out too