Agreed, ground freight. I worked in Munich for 18 months and had a lot stuff by the end of it, including items of furniture and so on. I got quotes and used a door-to-door service. That was 2001 so no longer relevant in cost comparison. Freight companies have regular shipments every week and so your goods get loaded into a van/container and off they go. they do all the paperwork and you pay them. Easy. The saving of time and effort alone, is worth the cost.
If your friend MUST take extra suitcases on the day - they should phone LH for accompanied air-freight (names vary). As I recall, you deliver the cases to the airport at least 24 hours before your flight. they then travel on your flight at a special rate. NOT excess bag rates.
Lastly. Have a friend drive over with a suitable vehicle BUT don't try one-way car/van hire as it's VERY expensive. Go the freight companies, there will be competition amongst them.