Having similar issues at the minute myself, actually. Just got an ADF on Saturday but I'm not sure he's eaten anything and the betta is getting too much. I have always feed my betta bloodworms with tongs, so they already know to look out for it and he's too quick for me to get anywhere near the bottom of the tank without him nabbing it (it's only a 5.5g). Also having a lot of trouble even locating the frog. I've a fair amount of plants and a decent sized piece of spider wood, all of which provide very effective cover.
That might be a blessing, though; Sagat's not as aggressive as Tong Po but neither are most piranha. TP will kill or be killed by anything that goes into his tank, for sure; Sagat's always seemed very calm by comparison.
Sagat's been stalking and flaring at the frog, though. I saw him try to bite at the frog's leg when it was poking out of a plant but the frog is unharmed; still got the full webbing and moving normally. I only saw the frog twice briefly for all of yesterday, though and I was sitting next to the tank most of the day.
I do have something to try, though; got a spring-loaded 'tube within a tube' device, so you can put stuff in the interior tube and then when you press a button, it's released. The betta hasn't seen it before, so hopefully he'll keep his distance but it's finding the frog in the first place that's the current challenge. He surfaces now and then for air, but that's a split-second jumping out of the middle of an anubias bush or something then away into the undergrowth again...
Unless you have at least three ADF you won't see them much. They require others of their kind for confidence because they are so shy.