Yes, minilop1 is correct in that an U/S of the heart is part of the diagnostic work up for Heart problems.Blood tests, an ECG and an echo-cardiogram are also usually needed. Is your Daughter's Rabbit Insured ? If not, then of course the financial aspect of any diagnostics is going to be a big consideration. Very few of us have a bottomless pit of funds, however much we may wish to be able to say that 'cost doesn't matter'. For most of us it does.
By establishing the exact problem causing the heart murmur, which is often a faulty heart valve, then there are some types of medication that can help control the symptoms and support the heart function. But of course drugs would not 'cure' the primary condition. They may buy the Rabbit more time though, time that is of good quality.
If a referral to a Specialist is an option then this may be the way forward in order to obtain as much detail as possible as to exactly what is wrong. Then treatment options are better tailored to those specific issues.
If a lot of diagnostics is not an option for you then your Vet may consider treating your Daughter's Rabbit with a drug that helps support the heart muscle- reducing the workload of the heart ie reducing the blood pressure and volume load on the heart. Thus possibly reducing the rate of deterioration.
The one thing to keep in mind is that as long as a Rabbit feels well within themselves, which in my experience almost all Rabbits with heart problems do, right up to the very end ; then the Rabbit is completely happy and unaware of any problem at all. They do not worry about anything other than how they feel at any given moment, they dont have any anxieties about how a Heart problem may effect them in the next hour, let alone the next year.
Quality of life is what matters to the Rabbit, not quantity. Of course we all want our Rabbits for be with us for many years. But in a few cases this is not to be, but we can make sure that every day they are with us is filled with lots of excellent care and love. I am sure that both you and your Daughter will make sure that is the case.