This may be a check your local listings situation in terms of handling it correctly. Assuming this was a high school game, I'd use the directives given in our association meetings:
- I'd inform the coach #6 is an illegal number.
- I'd ask the coach if there was a legal uniform available or tape available to temporarily change the front and back numbers to something legal.
- If either change was possible, I'd allow it to be done immediately without penalty and be sure the scorer(s) received notification of the change as required by rule.
- If neither change was possible, I'd remind the coach that being assessed a single direct technical foul is the penalty for players participating with illegal uniforms. I believe I'd also be required to submit an online report to the state association about the player participating with an illegal uniform.
I've only had this come up once when a team's new uniforms didn't arrive before the season started and they used taped-up numbers on their practice jerseys as they waited out the supply chain issues. Upon seeing #17 in warm-ups and the scorebook, I informed the coach #17 was illegal, but he could have the top part removed off the 7 which would give the player #11 - a legal number - instead. He chose that option, we made sure the scorers knew #17 was now #11, and the coach appreciated us being preventative.