Normally FCP will import AVCHD files by copying and re-wrapping them inside the library. Some people copy the .mts files outside the library bundle with Finder, then import them with “leave files in place”. That should never be done due to I/O performance issues.
In either case, simply deleting the references to those files within FCP will not cause red clips. If clips are red that implies he manually deleted the on-disk media files themselves with Finder.
FCP will never delete external media files and will only delete internal media files if no references exist, IOW the files are not used in any project (hence no red clips since they aren’t being used anywhere). Even in this case it warns you with the pop-up “Media Moving to Trash…One or more media files will be moved to the trash since the last reference to them is being deleted. External file will remain where they are”.
Due to inode resolution, FCP will even find external media files if all symlinks to those are deleted from within the library, if every media file is renamed and each renamed file is moved to another location on that volume. This assumes either a Mac OS Extended or APFS volume.
If the AVCHD file was internal and was deleted via Finder from within the library, and if you have the original non-rewrapped .mts file from within the AVCHD bundle, that can be relinked. In FCP find the red clip, select it, then pick File>Relink Files>Original Media and point to that file.
If the AVCHD file was extracted from the bundle with Finder and the bare .mts files imported with “leave files in place”, that file can also be relinked.
You can verify where FCP is looking for the media file by picking the red clip, do File>Relink Files>Original Media, then do not relink but look at the bottom of the dialog and in grey letters it will show the pathname where it expects the file.
If the machine has a Time Machine or any other backup type, as a contingency it might be good to preserve that before it is overwritten. The missing media files or earlier versions of the library before the problem might be preserved there.