There are new high effficiency boilers that are 1/3 to 1/2 the size but they do a bang-up job. There is no more comfortable or quieter heat than hot water as long as it has the air out of the lines. If she is not interested in air conditoning the forced air system would be a big disappointment; it won't save fuel any better than a new, high efficency boiler. Installing forced air would be a major job, taking out the radiators, plugging holes, getting rid of pipes, cutting holes for duct work; its a nightmare.
A hot water technician (hard to find) may be able to repair the leaks in the radiators and put in the new boiler. However, new radiators are also available.
I changed mine out for the forced air but I used a geo-thermal system so we could get central air too. It was still quite a challenge.