not from the way you've described it. You need to speak to the installer.
It is possibly to do with the distance from the flue to the boundary of your property, or maybe the position of windows.
Ask the installer.
When he has explained what the problem is, ask if a flue deflector kit would fix it. If not, ask why not. You can look up the manufacturer's instructions for the flue positioning on the installation instructions on their website. It is essential that you know what model of boiler you are paying for.
Also ask where the condensate pipe is going. If he says "outside" ask why it can't be run to a nearby indoor waste pipe, for example in the kitchen or bathroom, where it won't freeze in frosty weather and stop your boiler from working.
BTW I also suggest you look on the Vaillant website. See if they have an Approved Installer list for your area. See if your mum's chum is on it. See what guarantee they will give if an Approved Installer is used, and what guarantee they will give if not.
Use this website www.gassaferegister.co.uk/ to verify that the person who will install your boiler, whose name you know, is registered.
Finally, just for me, fill a bucket at your kitchen sink cold tap; time it; calculate how many litres per minute you get. Tell me. Also ask your mum's chum what the dynamic flow in your house is, and if the boiler he recommends will be suitable. If he says he doesn't know the dynamic flow, ask how he knows the boiler will be suitable (clue: He doesn't), and invite him to measure it before proceeding.