As long as they’re both of an age to consent legally, then yes, absolutely. Not sure what jurisdiction you’re in but 16 is age of consent in most places, and they’re going to do it if they want to do it. It’s far better to treat them like adults, let them be somewhere safe, and teach them that they should not feel shameful or fearful about that part of them. If the age of consent where you are is 18, or Tiffany is not legally of age, then the situation would be quite different and difficult but would have to be no.
That your son is able to be so honest and comfortable with you, and Tiffany with her parents (hopefully!) is a credit to you all. This is an opportunity to encourage him to practice safe sex and also that sex as part of a healthy relationship like this is healthy and valuable in itself (as opposed to sneaking around, and him not able or even bothered to provide a safe space for his girlfriend). It’s worth encouraging this attitude that to do this they should be somewhere safe, and comfortable, such as his room in your home. It will also ensure a much better experience for both of them should it be a first time (whether overall or just together).
I would just consider confirming that Tiffany’s parents are aware of this and comfortable with this; especially as at 17 are in a grey area socially between being legally adults in many areas but not others. EDIT: As someone else suggests, ask Tiffany to get her parents to call you - so that the ball is in her court.
I would also even consider being out until late so they can have some privacy. I would also set some ground rules such as no drinking or drugs in exchange for this, and no one else in the house while you’re out. I totally accept that may not instinctively sit well with you to be out but they’re both at the cusp of adulthood and total autonomy (literally months away if your son is 17) and already have autonomy in this decision (unless you’re in one of the few places where the age of consent is 18).
This sort of treatment, respect and trust is something your son will remember for the rest of his life.
I’m in the UK, and am paying £32 ($41) a month for 1Gbps symmetric fibre to the home.
Edit: worth adding my sibling is paying £36 a month for 1gbps down/100mbps up from the main network in the UK, Openreach (they run the old poles and other companies run services on their network). Thats probably the more common fibre package at the moment.
I’m on a newer independent network, and there are other companies building out independent telephone pole networks in the UK.
I used to be on Virgin cable and was paying £56 a month for 512mbps down, 100mbps up.