Sink drain pipe angle

All drainage pipework most definitely should flow downhill.
Any pipework that runs up hill, will most certainly, if not immediately, have problems in time.

Hiya
No you are right with what you are saying to him if the pipe does not have a fall then over time waist will just build up in that area and block
Hope this helps

No the pipe should definitely be angled for gravity to do its work and allow drainage preventing blockage should be set at 1in 40 aka 25mm drop over a metre

Your understanding is correct. In a typical drainage system, water should flow downhill due to gravity. If the sink drain pipe has a visible upward slope after the U-bend, it may lead to drainage issues. Water may not flow efficiently, and you might encounter slow drainage or potential backups.

It's advisable to have the issue addressed by the person who performed the kitchen renovation. A properly installed drain pipe should follow a downward slope to ensure effective drainage and prevent standing water. If you're experiencing problems, it's crucial to have a professional plumber assess and rectify the situation to ensure proper functionality and prevent potential issues in the future.

Water will pass through, however this will be a very slow process. This will result in the u-bend getting blocked sooner than it should. This is not a practice that we would follow when undertaking plumbing works.

If you are right, in a short time it will be filled with food and soap residue and will become blocked.

All waste pipes must be fitted with a fall so water can flow.

Yes and no. If the drain entry is higher, it will still drain, but it could accumulate solids and block easier.