You should be able to remove the broken nipple from the tee with a nipple extractor then replace the nipple.
Nipple extractor for removing the nipple that connects the sprinkler head to the pipe.
https://smile.amazon.com/Orbit-Sprinkler-System-Extractor-26073D/dp/B000BQRAOM
Definitely irrigation. I recognize the "nipple extractor" tool on the right.
https://smile.amazon.com/gp/product/B000BQRAOM/
Maybe landscaping in general (with lighting), but less clear.
Just grab it with pliers. You're throwing it away anyway, its soft plastic and not very reusable, and cheap to replace if you need too. If it breaks off, one of these is designed to extract it https://www.amazon.com/Orbit-Sprinkler-System-Extractor-26073D/dp/B000BQRAOM/ or a similar extractor, Lowes sells this type of tool for example.
The whole "soft plastic" thing is the reason to replace it. Not the end of the world if you can't remove it and plug the pipe, but risers are pretty much designed to break (to save the pipe underneath from damage) and so they are pretty weak and a potential leakage point.
There's a tool you get get that help grab st the inner pipe so you can twist it out. Orbit Sprinkler System 1/2-Inch Pro-Series Pipe Nipple Extractor 26073D https://www.amazon.com/dp/B000BQRAOM/ref=cm_sw_r_cp_api_i_ICVjFbSEA236M