Could be or it is spiking intermittently. If he has the ammeter he could see bounce a high speeds. My guess is a component short or wire rubbing. These will also show up on the ammeter if he watches closely.
On my Scag Wildcat the PTO clutch was worn to the point it needed adjustment. The fuses blew more often in thicker grass. I noticed the wiring to the fuse holder was also getting hot and even had to replace PTO switch. Maybe this will help..........
Get the serial number of your machine, go to the Scag website, click on manuals on the left, click on STT, find your serial number group, click on the parts manual. It will download as a pdf and toward the end of the manual are the wiring diagrams, print out the one that has your engine.
Generally when fuses get hot there are several causes; the voltage regulator in the regulator/rectifier is letting the voltage get to high. With the engine running at half throttle or more, the voltage across the battery should be not more than 14.3 VDC. All battery power passes thru the key switch and several connectors along the way, all of which are subject to high resistance from corrosion and dirt. The charging system output is also controlled by the keyswitch and when in the off position, the charging system is isolated so battery power won't back feed and drain the battery. Keyswitches are known to go bad. As has been mentioned, chaffing of wiring is always a possibility. Battery ground connections which are generally connected to an engine mounting bolt are another possible problem source. Why did you install a new clutch? Have you needed to recharge the battery between machine uses?
Check your manual as I don't know of any Scag machines that use a 30 AMP fuse, just 20s. You may need a new clutch as when there are electrical problems in the electromagnetic coil then amperage draw increases to the point where the charging system cannot meet the demand from the clutch.
Check out this attachment. The resistance in the clutch coil will tell you if it is out of spec. The lower the resistance, the higher the amperage draw. You more than likely have an Ogura clutch. Find the power plug for the clutch, disconnect it but be careful as it has a lock tab on it that must be lifted up a little to unlock it. Then you can check the resistance of the clutch coil.