if a user's ISP fiber connection is terminated with a single-mode simplex SC connector, what are their options for using an SFP/SFP+?

I've look around a bit, and I haven't found an SFP transceiver with an SC connector, they all seem to be duplex LC connectors...

Is there a way to adapt a simplex SC connector to a duplex LC SFP transceiver?

SC connector is too large for SFP module. So you need a converter.
If your connection is using single fibre strand (a.k.a. BiDi), then a simplex female SC to male LC adapter would do (beware that such connection will cause higher stress to SFP module connector). You'll have to use appropriate BiDi SFP connector (beware of proper Tx and Rx wavelength, these have to match optical transciever on network side).
For duplex (twin fibre strand) connection a female-female adapter seems a better choice (you'll have to use a short LC-LC patch cord between SFP module and this adapter).

FS offers a vide variety of adapters, so look around and choose the one which fits your needs best.

I have an old single fibre SC SFP. It's made by FiberXon.
Because it's quite old its probably not available anymore.

You need to know what type of optical network the ISP provides and where the demarcation point between their network and the customer is - often it is an ethernet socket on Network Terminating Equipment (NTE) or Optical Network Terminal (ONT). Their equipment may be necessary so they can carry out remote monitoring and diagnostics, in some cases it is possible to spoof their equipment.

I've come across 1000Base-BX SFPs with a single SC connecting leased line carriers access network to their NTEs, e.g. https://www.prolabs.com/products/transc ... 61003030-c, and PON SFPs can also have a single SC connector, e.g. https://www.fs.com/uk/c/wireless-access ... 805&page=1.