Frogspawn and Hammers are usually fine next to each other. The main thing is that unlike may corals, a branching hammer can lose a head without affecting nearby heads to any extent so no need to frag unless it gets progressively worse. The same can not be said for wall hammers that used to be very common in the hobby. They are not only very difficult to frag but losing a head almost always lead to losing the colony.
Is it near any softies or aggressive LPS?
I'll tell you what happened with my last 'hammer extinction event' as maybe there will be something in it that helps.
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I had three hammers, a green, a gold and a peach. The green and gold started as 2-head colonies and the peach started as a 3-head. They were all doing well for many months. At the time of the incident the green was about twenty heads, the peach was eleven and the gold was fourteen. The green hammer was at the far end of the tank and the gold and peach were both a few inches in front of the cave the Peppermints were calling home.
I added three Peppermint Shrimp for Aiptasia control and all was well. They started slowly but after a few weeks the Aiptasia population was noticeably diminished. Everything was fine for a while. I don't really remember how long. One morning I noticed two of the heads on the gold hammer were damaged and one was completely gone. It looked like the shrimp had damaged the coral but I had not fed that day. Since hammers are frequently 'checked' by shrimp and fish I cursed a little and forgot about it. Over the course of, I think it was three days, the shrimp killed most of the heads on the gold and four of the heads on the peach. I moved them both away from the cave. The next day two more heads of the peach were gone and two heads on the green and one damaged head on the green as well. I set up a 15 gallon tank that day and transferred the hammers to it.
The gold lasted a couple more days but just shriveled up and died. The peach took a while but the four of the remaining five heads began to improve. The green never looked bad it just lost two heads. I eventually caught and gave the Peppermints to an LFS. I did frag the peach at that time because one of the two main stalks was nothing but dead heads so I severed that stalk much like you proposed for your hammer. The peach and the green have been returned to the display. The peach is back up to seven heads and is still a little deflated but improving every day. The green never looked bad it just has two dead stalks that you can only see from certain angles. I don't know how many heads it has now but I would guess somewhere around 25-30.
Hope it helps.
