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?

You also need the correct thread sealant (pipe dope) for gas pipe, "Rectorseal" is the preferred brand of most contractors. 

I would hae been satisfied with the sealant that covers the surface and makes everything a uniform black color at no extra charge, which the contractor promied via text and an additional contract. 

I removed it all and put a sealant over it.

I would however pour in some waterproofing sealant on the floor to prevent upwelling of moisture. 

Quality means they cut a groove into the brick mortar and folded the flashing into the groove, with everything sealed up with silicone/roof sealant.

I also, gutted the semi-finished basement as there was wood paneling against cinderblock walls with no wet lock or sealant (causing white mold in some spots).

You can just get a tube of sealant and do it yourself. 

Seal the gaps and edges, make sure floor is levelish and free of sharp objects and debris every crawl has collectedAll in for everything, grading and plastic and sealant your looking at 500 bucks or LESS if you do it yourself and have a few tools and few hours of time.

Has anyone ever dealt with tar sealant (waterproofing, exterior grade) used on the INSIDE walls of the basement?