What happens if my husband has a nocturnal emission on the night before egg retrieval?

Let your doctor know. Men can usually collect a sample on the net day after a nocturnal emission. The sperm counts may be lower than expected, but the quality and performance of the sperm should be just fine. Icsi is always an option that can help.
Probable no effect. Unlikely to have significant effect, especially if semen analyses have been normal. Worst case, if spermatozoa count or motility is subnormal, then the option of doing icsi is available and the risk of having ivf cycle cancelled is quite small.
No worries. The emission before egg retrieval probably did not lower the sperm count or quality. Some men with poor semen can have a problem but then for ivf he would probably already need icsi. Rarely, we have men who come for ivf retrieval and the semen is really poor, so we suggest icsi (sperm injected into egg) even if not already planned. Icsi will allow normal fertilization. Hope this helps.