Silki, since it is not your website tell the owner to update it and solve the problems.
It is the owners responsibility, not yours.
If he wants to hire (and pay) you, it is up to you if you take the job or not.
Fact remains that the problem is with the website and not with avast.
Other av's just don't have the detection that avast has.
Look at what Polonus posted.
Be glad that you are using avast and not another av.
There is only one solution
Fix the problems with the website and avast will not block it.