I think I might re-try opening an attachment once. If it doesn't work, or gives problems perhaps because the file is corrupted somehow - it goes into the trash. THAT (crashing the Mail app) does not necessarily mean malware, or anything else, other than a file that you can't open. Maybe the file is just damaged.
Trash the file and move on. If your friend still thinks that is an important file, and can verify by contacting the sender that the file should be a good one, have that sender send that same file again, using some different process other than as an attachment.