Doncaster made a good start to Saturday's Championship contest and took the lead in the 32nd minute when John Oster played in Simon Gillett and he finished well for his first goal in nearly four years.
Hornets boss Malky Mackay brought Marvin Sordell on 10 minutes after the break and the change paid dividends when he headed home Don Cowie's cross with 63 minutes gone.
Seven minutes later, the impressive Sordell got on the end of goalkeeper Scott Loach's long punt upfield and fired past Neil Sullivan to put Watford in front.
It looked like that would be enough to get Watford the three points but two minutes into injury-time Martin Woods' corner found Friend and he stabbed in a dramatic equaliser.
Watford made a bright start and Jordon Mutch tried his luck from 20 yards but his shot fizzed wide.
The Hornets could have gone ahead after nine minutes when Cowie's corner found Martin Taylor but he headed over from close range.
Doncaster eventually began to find their rhythm and Billy Sharp's 20-yard effort went wide with Loach untroubled.
Minutes later, Woods fired in another shot from distance but it went just over the bar.
Neither side were managing to fashion any clear chances on goal and Watford midfielder Mutch sent in a free-kick from 25 yards that went just over.
Watford began to put the Doncaster defence under pressure but the visitors took the lead with 32 minutes gone after an incisive break.
Sharp held play up superbly before passing to Oster and he played in Gillett who kept his nerve to produce a fine finish past Loach.
The home side pushed forward in search of an equaliser before the break but the closest they came was a weak shot from Mutch that went wide.
Watford came flying out the traps at the start of the second half and Danny Graham saw a shot from the edge of the box parried away by veteran Doncaster goalkeeper Sullivan.
Mackay brought Sordell on for Stephen McGinn with 55 minutes gone and eight minutes later the substitute drew Watford level.
Cowie got down the right and crossed for Sordell who got in front of his marker and headed past Sullivan.
With 20 minutes remaining, Sordell grabbed his second when he latched onto Loach's long kick out and held off the challenge of Gillett before finding the bottom left-hand corner.
Watford continued to press in search of a third goal and Sullivan did well to parry away a shot from Graham.
And Doncaster duly made Watford pay for not putting the game to bed when Friend got on the end of Woods' corner and poked the ball home.