So my friend recently bought a house but unfortunately didn't do a sewer inspection on the main line before purchase. Having had one done recently with a camera inspection on the main line, the inspecting plumber noted that the pipe had tree roots on several parts of the mainline. The plumber recommended a sewer trenchless replacement. They also mentioned they would seal the replacement pipe with some kind of "sealant" so that tree roots couldn't get in their again.

My friend was also planning to build an accessory dwelling unit (ADU) in their backyard.

A few questions:

1) Should my friend have the sewer pipe replacement done with the ADU construction (thinking it might result in a lower overall total cost for all the work done) or should they hire out the sewer replacement job separately? And how soon should it be done?

2) What's a reasonable estimate for this type of work? (they live in Southern California, Ventura County - Simi Valley/Moorpark area;  you can think Los Angeles if you're not familiar with Ventura).

3) Is there any other part of the sewage system of the house that can or should be inspected

4) What is the name of the "sealant" for the new pipe that the plumber was referring to so I can compare brands/solutions?

For additional context:

There doesn't seem to be an immediate problem with sewer blockage. The only thing my friend noted is that when there was a heavy rain, and they did laundry, water backed up into the master bedroom shower and the hallway bathroom bathtub. So clearly there's a problem, but on other days there's been no issue so far.