Asus routers have a guest network. My guess is that there's plenty to choose from, it's a case of getting the best you can for the budget. Before I upgraded to fibre I used an Asus N55U on ADSL and the Wifi performance was quite good.
If the budget is tight it's a good idea to look to see what's going on Ebay then check the spec from the manufacturers web site and to read performance reviews. Take into account how old the review is as what was considered good a few years ago may be no more than average these days.
The other thing is for your friend to consider future requirements and maybe spend a bit more and get one that is compatible with both ADSL and VDSL standards. If customers do start using a guest network in earnest he might come under pressure to have more bandwidth available than ADSL2+ can provide and this way would be ready to upgrade to fibre without the need to buy more hardware.
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