Good Morning "M",
Well, the bad news is there are virtually no trails in the Catskills that are perfectly flat with no inclines. The Catskills (by nature) are mountainous with steep trails and even near-vertical ledges requiring the use of both hands and feet. If your friend has a "bad leg" and cannot do inclines you may wish to "re-calibrate" your plans. Besides the difficulty of hiking (even with a pair of strong healthy legs), you also need to consider how you will quickly evacuate your friend if there is a medical or weather-related emergency.
The only trail that even comes close to being "level" and scenic would be the Escarpment Trail around North/South Lake. However, even that trail has ledges and steep scrambles that may be too difficult for your friend. The only other "possible" option I can think of is the Spruceton Trail from Spruceton Road to the summit of Hunter Mountain. The trail is actually a jeep road and is mostly a gentle ascent to the summit of Hunter. About two thirds of the way to the summit is the John Robb Lean-to which provides both a camping area and a breathtaking view of Spruceton Valley. However, if your friend cannot do inclines at all, then even the Spruceton Trail may be too difficult.
As for bears, they live everywhere in the Catskills in substantial numbers and the odds are fairly good you will encounter one. I have had several bear encounters this year as their populations have increased. However, the good news is they are generally very docile, and are very shy and timid around humans. They will usually turn and run at the first sight or sound of a person. Unless you do something truly stupid (e.g., approach the bear and try to feed it or pet it - yes, people do that!) you have no reason to be fearful of bears.
I sincerely hope my comments have not "let the air out of your balloon" about your plans. The Catskills are a wondrously beautiful place but the Catskills can also be dangerous. You need to carefully plan your visit and be realistic in terms of the abilities and limitations of both yourself and your friend.