If you are running a service or a platform (e.g. operating system), then MTTF and MTBF are very important factors in software and have been used for quite some time to assess reliability usually via some form of stress testing.
One of the primary purposes of stress testing is to find the MTTF (mean time time failure) which generally occurs due to memory leaks, resource capacity, bandwidth, etc. Simply stated, stress testing can help us determine the average amount of time our software is used before it crashes or hangs.
MTBF is esp. important for services and servers, so companies know when a server goes off line how many seconds, minutes it will take to 'reboot' the software (assuming no hardware failure).
Being able to predict MTTF and MTBF helps services load balance and also perform scheduled maintenance.
That being said, many people often confuse MTBF and MTTF. So, if you don't understand what your clients want you shoud ask them to clarify how they will use that information. I suspect they are likely looking for reliabilty data, of which MTBF, MTTF are just 2.