Yeah, it's generally not advisable to mix speeds from a stability standpoint - although it would be easier to make a judgement if it were known that the same brand/configuration of chips were used in both DIMMs. I've had some systems that ran great with a mix (albeit at the slowest set's speeds) but I've had others that had stability issues that I could only trace to differing timing and voltage requirements.
I'd say to be safe you would want to manually set the same timings (and a relaxed set of those at that) for all of them in BIOS and also the voltages. If you leave any of those settings at "AUTO" then it's up to the motherboard to figure it out based on the tables in the RAM itself - and those might not play well with each other.
In short, if this were your
system... I would say go for it and just tweak as necessary. Considering this is a system you hope will not require any support after it's handed off - I'd have to recommend using all the exact same RAM (brand,speed,timings, and preferrably even production batch). Even if you set everything up in a way that works well manually... what happens when they decide to clear the CMOS or update to the latest BIOS version (although if they are doing that
then they should know what they're doing enough to figure out RAM issues as well).