It takes them a while to adjust to frusemide. In general take them out several times after they've had the pill, for the first 90 minutes. Definitely get them out 30 minutes after the pill. Never give the pill less than several hours before bedtime so that again, you can take them out and their bladder is very empty overnight. I'd also only give it when you will be around for the next 90 minutes or so.
Leo was on frusemide for SM and it took him a week or two to realise he had a very full bladder right after. Sometimes he'd wet himself at night too. Leo could eventually last about 60 minutes before he had to go out but I wouldn't wait that long with an elderly dog and would get her out several times. She may not learn to go out on her own as they don;t expect to suddenly get such a full bladder. Frusemide is a diuretic -- like drinking a few cups of coffee for us. That is why it gets rid of the fluid that causes the cough. You just need to plan around the fact that she is going to need to go out a few times in that first 60-90 minutes after the pill.