I would take off the distributor cap and check it out along with rotor to see if it's cracked or in bad shape. Lots of guys have solved their start problem there. Also check plug wires (about $100.00 for replacement set). These cars take $2.00 copper plugs and run great with them. If it's turning and cranking, you have a chance. Many owners throw $$ at fuel pumps, relays, filters, etc. and nothing. You may end up throwing parts at it and eventually get it to start but could cost. Dying issues (usually just turns right off or won't start at all) while driving are associated to the fuel relays which you stated he replaced already, but try to reseat or wiggle them (located behind the battery, behind the plastic partition that slides up and out). This corrected mine and my daughter's 300E starting and dying issues. No problems for over a year now. 1991 300E 226K.

Good luck.