Do you have a combustible gas detection meter? They are not that expensive, and a good tool to have in your bag.
Use that to try to find the source(s) of the odor. Dont rule out that you may have more than one source.
Correct me if wrong, but I am assuming the sewer pipe repairs involved cutting the basement concrete floor open to access and repair piping? If a exosed soil floor, that is important to know.
Odor could be due to Soil gas from decaying organics under the basement slab (I assume you have an enclosed basement). Check the slab to wall corner joints and any other cracks or joints in the slab with the gas leak meter for elevated readings. If sewage leaked during the recent sewer repair work, it may be just under the slab and the odor may be seeping through the open joints in the floor.
Make sure the sewer repair work area was thoroughly cleaned after repair was completed. Workers make a big mess, and may not have cleaned up all sewage spills well.
Is the basement ceiling finished? If not, use the meter to sniff all the drain piping. Your looking for cracked sewer pipes, whether they serve as drains or vents. Cracks or open joints may be at the top of the pie, so you won’t see water leaks , but gas can get out.
Did you plumber get permits and inspection by building department? Did they make him test the piping after repair? Check for test plugs left in roof top vent pipes… sometimes they forget to remove them.
Is there an exhaust fan in the bathroom? Should be, or a window that opens. Make sure the back draft flap is sealing properly. Check the fan for odors with the meter.
It also may simply be that your client has a highly sensitive sense of smell and is detecting an odor which has nothing to do with your sewer system. He might want to try having the entire basement professionally cleaned, deodorize, disinfected, and then Place some damp rid type devices down there control humidity levels. Elevated relative humidity has a musty odor to it that people commonly mistake for sewer gas smells. Place some relative humidity detectors in the basement to make sure the humidity is below 60%.
If the relative humidity is above 60% then mold can grow on surfaces, and mold in pain to have bad smells as well. Cleaning and disinfecting regularly and adding a dehumidifier generally controls those issues.
There could be several other possible explanations those are the most likely.
When investigators holders that are intermittent, I always encourage my clients to call me Community is bleed when they smell it oh, and if I can I will run over and check it out while his the smell is present. It’s very difficult to troubleshoot this type of problem when you can’t even smell it.