Putting opponent’s cards in your hand in canasta
The Official Rules of Canasta describes optional point penalties for certain Irregularities in the play.


  Point Penalties
  
  
    Penalties paid in points, rather than by forced plays that rectify the error, are unpopular with many players. Therefore players may choose either to use or to omit the following point penalties, In all cases these penalties are additional to the penalties prescribed in rules 51 to 60.
  


One of these is for illegally taking the Discard Pile, which seems to clearly apply here:


  
    For taking the discard pile into the hand illegally, 200.
  


As play on the hand cannot possibly continue, and neither can it be scored in any meaningful sense, it should be redealt by the same dealer after applying the Point Penalty above. THis seems a natural extension of Rule 52:


  52. There must be a new deal by the same player if it is ascertained, before each player has had a turn to play, that a card is faced in dealing or found faced in the deck; or that the dealer departed in any respect from the rules of correct procedure in dealing; or that a player was dealt an incorrect number of cards. After this time limit, the deal and any incorrect hand stands as regular.


The Official Rules of Rubber Bridge note two points which I believe are pertinent, and should apply to all attempts at remedying irregularities of any card game:


  The Scope of The Laws
  
  The Laws are designed to define correct procedure
  and to provide an adequate remedy whenever a player
  accidentally, carelessly or inadvertently disturbs the
  proper course of the game, or gains an unintentional
  but nevertheless unfair advantage. An offending player
  should be ready to graciously accept any penalty set
  forth in these Laws or any adjustment or decision of an
  Arbiter.
  These Laws do not deal with dishonorable practices
  where ostracism is the ultimate remedy.
  
  1. General Principles:
  
  These Laws cannot cover every situation that might
  arise, nor can they produce equity in every situation covered.
  Occasionally the players themselves must redress
  damage. The guiding principle: the side that commits an
  irregularity bears an obligation not to gain directly from
  the infraction.


I would extend this to say that an egregious offence, even when inadvertent, should always carry a penalty when it is impossible to restore play - in order to prevent even the possibility of temptation to commit such an offence intentionally. Further, to be so careless as to pick up an opponent's hand when picking up the discard pile is exactly such an egregious offence - as it could otherwise be used to force a likely redeal when the current situation is disadvantageous.

Yes, Rubber Bridge is a different game; but carefully note this commentary on the Canasta Rules:


  
    "The Canasta procedure and Irregularities as described here, and adopted by the Association of American Playing Card Manufacturers, are the same in substance as the rules given in the books of Oswald Jacoby, Ottilie Reilly, Albert H. Morehead, Geoffrey Mott-Smith, Ralph Michaels, Charles Goren, Fly Culbertson, Richard L. Frey, Samuel Fry, Theodore A. Lightner, and others, and endorsed by the Regency Club of New York and other clubs."
    
    Arranged by Albert H. Morehead.
  


All of Oswald Jacoby, Ottilie Reilly, Albert H. Morehead, Geoffrey Mott-Smith, Ralph Michaels, Charles Goren, Fly Culbertson, Richard L. Frey, Samuel Fry, and Theodore A. Lightner were world class bridge players as well as Canasta experts. Both Morehead and Mott-SMith were at one time chairman of the ACBL Laws Commission. They applied the same standards of fair play to all the card games which they played and excelled at.