Go for the work, you are not poaching, they have approached you, not the other way around.
Find out what he was charging, and then make sure you charge more though.
I wouldn't bother talking to him as that makes it more personal, if he then objects to you taking the work it becomes more difficult to do so, and you risk it becoming very acrimonious too.
Prices for shop fronts?
As Roger says, for a single big window (or maybe 2) and a door, then aim at a minimum charge of something like £2.50
If they have a window cleaner it is highly unlikely (particularly if it is in a village) that they will be paying much more than that.
If you can pick up a few shops, then even at a paltry £2.50 a go it works out as good money.
The thing is, shops are very often highly sought after and very competitively priced.
If you have a single large pane of glass and a door then it is no more than a minute and a half to do.
It's a catch 22 situation, they are a bind, you have to be as regular as clockwork, one or 2 shops are a nightmare as the travelling involved is a real pain.
To travel even a single mile to clean a single shop for just £2.50 is a dead loss, but getting more than that is not easy.
If these shops are done by the same window cleaner and they are unhappy with him, then with any luck you will be able to find out what he was charging, and base your own price accordingly, and if they are unhappy with him and his work, then you will be able to hike the price a little.
You may be lucky and the shops in that area are having to pay a decent rate
On Monday I cleaned 38 shops/pubs/offices etc. six of them were £2.50, Seven of them were £3.50, the most expensive being a building society for £18 and a large shop for £25
So as you can see, the spread of prices can be enourmous.
You really need to find out the kind of prices that are currently being charged in your area.
Ian