My neighbor down the street, said they have a mercury vapor light that has been having issues. They said it stop working so they replaced the bulb and the photocell and that it only flickers. I'm wondering if they didn't mean a sodium light, because I don't know of any MV lights at their place. However, if it is MV I don't know what the issue could be, but if it's HPS I'm thinking maybe a bad ignitor.

They wanted to know if any other things to check but all I can think is ignitor on an HPS fixture.

It could also be a metal halide fixture.

Bad ignitor would just not let the lamp start.
Flickering used to originate from distorted mains voltage (even harmonics and mainly subharmonics, typical from some large load with bad contacts on the same circuit).
Or some ballast component may start failing (flashovers inside of the CWA capacitor,...).
Or the lamp post is vibrating, causing irregular gas movement within the arctube.

I've seen a few cases where an MH or HPS lamp will just flicker while the ignitor tries to start it but is unable to due to the lamp being defective.

Now I am not sure, since I know this neighbor had an HPS bucket light, but I noticed last night it was working, so they must be talking of a different light. Unless that is the one they meant, and it's now fixed. In their back yard, they have a GE Town and Country style lantern, that I haven't seen turned on in a long time, but it was HPS also.

Merc lamp on HPS gear ? (especially since some HPS ignitors indeed work only in one half cycle)

Bad electronic photocell ? (if the switching component is a Triac, perhaps one half of it died so it rectifies like a diode)

"Bad connection" flickering caused by a bad connection ?

They said they replaced the photocell. It could be fixed already, not sure. Loose connection sounds possible also. I was a little confused because they said mercury vapor, but both lights I know of are HPS unless there's another one.

Is it working? If not tell them they might need to return the MV and buy an HPS lamp.